Ok, now, in the wonderful world of small Japanese cars with 4 cylinder engines, backpressure is a must for torque (unless you have a forced induction car, then who cares about back pressure). Why? I really don't know. But I was discussing this with my friends and we figured that when I removed the post-cat I probably lost 2-3 low end torque for about 2-3 high end horse power. My question is, what's the best way to keep the decent amount of back pressure while still having a decent amount of horse power gain on a NA car? Turboing will just be to expensive, I'd rather go with header, camshafts, and pistons.
Backpressure anyone?
- Thread starter Puccio
- Start date
mazdamp3_18
Member
- :
- 06 gti, 06 evo IX
personally im doing the same thing your doing, pistons, header, exhaust, intake, cams, and safc.
i would keep the 2nd cat and get rid of the first one when you get a header youll gain a lot of hp and torque when u do that. im actually keeping my stock muffler and increasing he diameter of the piping from the 2nd cat to 2.25 inches.
i would keep the 2nd cat and get rid of the first one when you get a header youll gain a lot of hp and torque when u do that. im actually keeping my stock muffler and increasing he diameter of the piping from the 2nd cat to 2.25 inches.
This is how I understand it:
Exhaust gasses escape from the cylinders in pulses (High pressure). As each pulse travel through the exhaust pipes, it creates an area of vaccuum behind it (Low pressure). Since gas travels from area of high pressure to low pressure, the greater the difference in pressure, the faster it'll travel.
The speed of the pulses are limited by the size of the pipes. If a pipe is too big or too small, the pulses will slow down causing less of a vaccuum behind it which in-turn minimizes the pressure difference. This in effect slows down the excavation of exhaust gasses from the cylinder and causes loss in HP.
Exhaust gasses escape from the cylinders in pulses (High pressure). As each pulse travel through the exhaust pipes, it creates an area of vaccuum behind it (Low pressure). Since gas travels from area of high pressure to low pressure, the greater the difference in pressure, the faster it'll travel.
The speed of the pulses are limited by the size of the pipes. If a pipe is too big or too small, the pulses will slow down causing less of a vaccuum behind it which in-turn minimizes the pressure difference. This in effect slows down the excavation of exhaust gasses from the cylinder and causes loss in HP.
Yes, anything that moves causes low pressure. It is the Burnouli (spelling?) effect. The best way to explain it is a shower. Have you ever been in the shower and the curtain comes in on you? That is cause the moving water from the shower head creates low pressure and pulls the curtain in. This is also how a plane flies and a curve ball works.
With pipe diametar, the smaller pipe will force the gas to flow out at a higher speed. A larger pipe, the gas will flow out easier but at a slower speed. The faster gas speed creates an area of low pressure behind it. That low pressure pulls the exhaust out of the cylinder on the next exhaust stroke, which makes for less work the piston has to do, which adds torque.
Hope that helps.
With pipe diametar, the smaller pipe will force the gas to flow out at a higher speed. A larger pipe, the gas will flow out easier but at a slower speed. The faster gas speed creates an area of low pressure behind it. That low pressure pulls the exhaust out of the cylinder on the next exhaust stroke, which makes for less work the piston has to do, which adds torque.
Hope that helps.
really, the whole shower trick has to do with the hot and cold areas next to the shower, which causes the curtain to creep in at you. as the hot water heats up the air in the shower, it lifts up and sucks coler air in at the bottom. this cold air that comes in pushes the curtain out of the way.
as far as back pressure in your exhaust, you want low back pressure, but high velocity. that is why high performance exhaust systems have mandrel bends and straight thru mufflers, to reduce back pressure. however, the quality systems are the ones that are tuned in diameter. a 4" exhaust system won't do you any good on a 1.6 liter civic, and a 2" exhaust system won't do you any good on a 5.0 liter mustang.
backpressure is a bad thing if you want power, because it will make the engine do work to push the exhaust out, which takes away from the work the engine is doing to turn the wheels. on the 2.0 liter protege's, i would think the best exhaust setup would be a 4-2-1 header, high flow cat, with a 2-2.25" mandrel bent exhaust to a straight thru muffler.
of course, i'm no scientist, but i did stay at a holidy inn last night!
NOO...
The curtain is the bernouli effect. Any fluid (gas or liquid) that moves causes an area of low pressure around the fluid. An airplane wing for example. The air hits the wing and rushes up over the top of the wing. This creates an area of low pressure above the wing that picks up on the wing, causeing the airplane to fly. The hot and cold water thing might cause it some too, but it is mainly the moving water. Go home and run a cold shower and see if you get the same thing.
I took Physics I and II in HS and 3 courses of physics in college, and I have never stayed at a Holiday Inn hehe
The curtain is the bernouli effect. Any fluid (gas or liquid) that moves causes an area of low pressure around the fluid. An airplane wing for example. The air hits the wing and rushes up over the top of the wing. This creates an area of low pressure above the wing that picks up on the wing, causeing the airplane to fly. The hot and cold water thing might cause it some too, but it is mainly the moving water. Go home and run a cold shower and see if you get the same thing.
I took Physics I and II in HS and 3 courses of physics in college, and I have never stayed at a Holiday Inn hehe
Originally posted by NC_MP3:
<STRONG>NOO...
The curtain is the bernouli effect. Any fluid (gas or liquid) that moves causes an area of low pressure around the fluid. An airplane wing for example. The air hits the wing and rushes up over the top of the wing. This creates an area of low pressure above the wing that picks up on the wing, causeing the airplane to fly. The hot and cold water thing might cause it some too, but it is mainly the moving water. Go home and run a cold shower and see if you get the same thing.
I took Physics I and II in HS and 3 courses of physics in college, and I have never stayed at a Holiday Inn hehe
yeah, i agree with the wing thing and flight and all that because i took all that s*** in school too. my first three years of college were spent majoring in physics, and then i transfered into mechanical engineering school for two more years. and the last time i ran a cold shower, nothing happened. at least untill my wife flushed the toilet on me and the water scalded my back...
oh, and the curtain attacked my leg.
[ 02-28-2002: Message edited by: Nik ]
Comming from an Engineering major, both of you are right. It has a little to do with both low pressure and thermodynamics. But to cause a low pressure area, a viscous fluid(air) has to flow from a large area into a smaller area in order to create increased speeds, and hence, a low pressure area.
deepcover6
Member
Just a little extra something to illustrate the point further....
The Venturi Effect
Discovered by observation by Daniel Bernoulli, a Swiss mathematician and physian in the 18th century. Fluid flowing through a passageway accelerates as it passes through a constriction which also causes a drop in pressure at that point. The Bernoulli Effect, as it is more correctly known, is responsible for aerodynamic lift. The profile of an aerofoil accelerates air flowing over its upper surface compared with that of the lower producing a lower pressure on the upper surface. Taken together, and acting against gravity, the effect is known as lift.
The Venturi Effect
Discovered by observation by Daniel Bernoulli, a Swiss mathematician and physian in the 18th century. Fluid flowing through a passageway accelerates as it passes through a constriction which also causes a drop in pressure at that point. The Bernoulli Effect, as it is more correctly known, is responsible for aerodynamic lift. The profile of an aerofoil accelerates air flowing over its upper surface compared with that of the lower producing a lower pressure on the upper surface. Taken together, and acting against gravity, the effect is known as lift.
OK, lets say it so some of the non engineering people can understand it a little better.
If you remove your cats, you will lose back pressure. In other words you will lose torque by gutting out your exhaust. You will however gain top end HP by doing due to the engine being able to breathe better on the top end. If you're really interested in learning about how exhaust systems work, just take a trip to borders or a library.
If you remove your cats, you will lose back pressure. In other words you will lose torque by gutting out your exhaust. You will however gain top end HP by doing due to the engine being able to breathe better on the top end. If you're really interested in learning about how exhaust systems work, just take a trip to borders or a library.
Have you guys done any of these things to know the difference from the seat of your pants? I understand that you can sit here and babble about viscus gases and high pressure low pressure s***.. Hey, i took off the rear cat and resonator and ran straight 2 1/2 all the way back to the muffler and it made quite a difference. This nice thing about this info is that I did it, and I felt it!!!
georgecastaneda
Member
- :
- 2000 Mazda Protege ES 1.8L
I have on my 2000 stock headers and pipe but i have a 4.25'' muffler i had to put it cuz i couldn't afford the stock muffler at that time but is that big muffler doing any good on my engine?
thanks
thanks
georgecastaneda
Member
- :
- 2000 Mazda Protege ES 1.8L
i have no other mods besides the CAI
CircuitRacer
Member
- :
- 2003 Mazda Protege 5
If you just welded in the muffler it really isnt gonna do anything besides change the sound.
georgecastaneda
Member
- :
- 2000 Mazda Protege ES 1.8L
it is welded since i didn't have any other stock muffler at that time, does that muffler affect my engine in any way ?
Last edited:
